Known processes for producing polymer latices of spherical particles include emulsion polymerization and suspension polymerization. In general, the emulsion polymerization technique is able to produce monodispersed polymer latices having a narrow size distribution by properly selecting conditions, but the particles have a maximum particle diameter of about 0.5 .mu.m. It is known that the particle size of polymer particles obtained by emulsion polymerization can be increased by (1) decreasing the amount of an emulsifier, (2) decreasing the amount of a polymerization initiator, or (3) adding an inorganic electrolyte. However, these techniques are disadvantageous in that emulsion polymerization stability is reduced and that the resulting polymer particles, though having an increased size, would be polydispersed.
In order to overcome these disadvantages, it has been proposed to control the concentration of the inorganic electrolyte as disclosed in Japanese Patent Application (OPI) No. 22904/84 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") or to use a solvent in combination with water in the production of a styrene-acrylamide copolymer latex as taught in Kobunshi Gakkai Yokoshu, Vol. 34, No. 2, 148 (1985).
However, the method of controlling the concentration of the inorganic electrolyte failed to obtain polymer particles of 1 .mu.m or greater. The method of using a combination of a solvent and water succeeded in obtaining a styreneacrylamide copolymer latex having a particle size of 1 .mu.m or greater but is inapplicable to polymerization of monomers other than styrene and acrylamide, proving unfit for a wide variety of uses.
On the other hand, according to the suspension polymerization technique, polymer particles having a broad range of size of from several micrometers to several millimeters can be produced, but it is impossible to obtain particles of uniform size.
It has also been proposed to obtain a monodispersed polymer by seed polymerization as described in Japanese Patent Application (OPI) No. 106554/83. This process comprises preparing fine polymer particles having a particle size of about 0.5 .mu.m by emulsion polymerization and then allowing the fine particles to grow.
Since the aforesaid seed polymerization starts with an emulsion-polymerized latex having a particle size of about 0.5 .mu.m, two or more steps of seed growth would be required to reach a particle size of from 5 to 10 .mu.m. This increase in the number of steps required makes the production facilities more complicated and increases costs. Further, solution polymerization in a heterogeneous system (hereinafter referred to as "solution system dispersion polymerization") is known as proposed in J. Polym. Sci. Polym. Lett., Ed., Vol. 23, 103-108 (1985), in which a one-step polymerization is carried out in a solvent capable of dissolving a monomer(s) but incapable of dissolving the resulting polymer to thereby obtain monodispersed fine polymer particles of micrometer size.
Utilizing the solution system dispersion polymerization method, polymer particles having a maximum size of about 10 .mu.m with a narrow size distribution can be produced. However, the size distribution is subject to great variation with changes of polymerization conditions, such as the polymerization temperature, the amount of polymerization initiator, and the like. Such a polymer latex is particularly unsuitable for use in the field, needing control of rheological behavior, such as an electrophotographic toner. The use of a solvent gives rise to a safety problem as well as the necessity for additional equipment for recovery of the solvent, etc.